The present invention generally relates to the tightening of bolted joints, and more particularly, to the uniform and accurate tightening of bolted joints formed with multiple fasteners.
The joining of components in any of a variety of industries often requires the development of bolted joints for effectively securing the components to each other. This can include any of a variety of complex assembly procedures for properly securing a series of fasteners associated with an assembled component or combination of components. Examples of such procedures can include applications such as the joining of cylinder head assemblies to cylinder blocks, which is common practice in the automotive industry, the joining of pipe flanges, having applicability to any of a number of industries, and the complex assembly procedures that are prevalent in the aerospace industry, among others.
Irrespective of the application involved, the overall goal is to achieve a substantially uniform load in all of the fasteners associated with a particular bolted joint being produced, in order to provide a proper connection of components, while performing the required tightening sequence in the least amount of time possible. Although the problems in achieving such a result have been known for some time, numerous attempts at solving such problems have not been entirely successful.
As an example, and for applications involving the connection of flanged joints, U.S. Pat. No. 5,278,775 (Bibel) discloses a method for tightening the threaded fasteners associated with the flanged joint in an effort to achieve a substantially uniform load in all of the fasteners associated with that joint. The disclosed method attempts to solve problems noted in Bibel, G. D., “Tightening Groups of Fasteners in a Structure and the Resulting Elastic Interaction”, Handbook of Bolts and Bolted Joints, Chapter 24, Marcel Dekker Inc. (1998), which recognizes that when a group of fasteners is tightened to form a joint, elongation of the individual fasteners causes structural interaction with the assembled joint which is being compressed, and that subsequent tightening further compresses the joint, reducing the preload in the previously tightened fasteners. Such effects are commonly referred to as “elastic interaction” or “bolt cross talk”. Another effect to be taken into consideration, which is commonly referred to as “rocking”, is where the load increases in a fastener diametrically opposite to the one being tightened. Such rocking can occur in a flange joint when the gasket outer diameter is smaller than the bolt circle diameter, which is often the case.
In an effort to accommodate such conditions, the method disclosed in U.S. Pat. No. 5,278,775 initially tightens each of the fasteners associated with the flanged joint system to a predetermined initial load or stress, in a first pass, and the final load, stress, strain or elongation is measured in each of the fasteners after all of the fasteners have been tightened. As used herein, a “pass” refers to a tightening procedure in which all of the fasteners for developing an assembled joint have been tightened once. Interaction coefficients representative of elastic interactions occurring between the fasteners in the system are thereafter calculated, and are used to predict an initial fastener strain value or load for each fastener in the system. These predicted values, together with the calculated interaction coefficients, are then used to tighten the threaded fasteners in a subsequent pass, whereupon the calculations and predictions are updated to achieve a desired tightening of the flanged joint.
Nevertheless, and even with load indicating fasteners such as the “I-Bolt®” fasteners which are available from Load Control Technologies of King of Prussia, Pa., it has not previously been possible to reliably achieve a satisfactory flange joint having substantially uniform stress on each of the fasteners without employing a significant number of passes in which each of the series of fasteners is sequentially tightened in a predefined pattern, resulting in a significant amount of time to produce the desired flange joint.
While the foregoing discusses problems associated with the joining of flanges, similar problems are presented in other complex assembly procedures. Moreover, such problems can further be complicated by the use of various different gasket materials for developing gasketed joints.